An increasing amount of vehicular traffic in the world's airways, highways, and waterways has unfortunately resulted in increased potential for vehicle collisions with other vehicles or fixed objects. U.S. and foreign governments, as well as industry, have recognized that an anti-collision system that actively prevents collisions from occurring presents the greatest potential for improving the safety of transportation. An anti-collision system would ideally alert vehicle operators to the likelihood of a collision in time to allow the operators to take evasive action designed to prevent the collision from occurring.
The problem of vehicle collisions is especially acute with aircraft, as there is usually a disastrous loss of life and property. In an effort to prevent aircraft collisions, the Federal Aviation Administration (FAA) mandated that by the end of 1993 all aircraft with thirty or more seats should be equipped with collision avoidance equipment. In response to this mandate, a family of airborne devices having a range of collision avoidance capabilities known as Traffic Alert and Collision Avoidance Systems (TCAS) has been developed.
The commercial TCAS that have been developed and adopted by the FAA for implementation within the National Air Traffic Control System rely on the inclusion and continuous operation of Air Traffic Control Radar Beacon System (ATCRBS) transponders in all private, commercial and military aircraft. When one TCAS equipped aircraft approaches a second aircraft, the method of establishing collision avoidance communication depends upon the type of ATCRBS transponder contained in the second aircraft. The first aircraft determines the location of the second aircraft by transmitting an "all-call" interrogation signal every second. If the second aircraft is equipped with a Mode A ATCRBS transponder, the ATCRBS transponder receives the interrogation signal and responds with a signal which allows the first aircraft to calculate the range and bearing of the second aircraft based on the direction and relative strength of the signal. If the second aircraft has a Mode C ATCRBS transponder, it can include altitude information in its response.
In contrast, if the second aircraft is equipped with a Mode S ATCRBS transponder, the method of interrogation by the first aircraft is slightly different. A Mode S ATCRBS transponder automatically transmits a message containing the "address" of the transmitting aircraft once a second. The TCAS equipped aircraft sends a direct interrogation to the second aircraft, based on the known address of the second aircraft. The Mode S ATCRBS transponder of the second aircraft responds with a signal which includes altitude information. To minimize interference with other aircraft, the rate at which a Mode S aircraft is interrogated depends on the range and closing speed of the two aircraft.
While the interrogation/response is somewhat effective at establishing the relative position of the second aircraft, it also has some undesired consequences. The first or interrogating aircraft also receives messages from all other ATCRBS transponders within radio range, requiring the TCAS equipment to use special techniques to filter out the unwanted signals. Furthermore, the strength of the response signal is potentially inconsistent due to variations in ATCRBS transponder equipment and antenna installations in different aircraft resulting in imprecise data on which to base collision avoidance calculations.
Regardless of the type of transponder contained in the interrogated target aircraft, the TCAS equipped aircraft uses the received signal to determine whether the flight path of the target aircraft is a potential threat. Depending upon the complexity of the TCAS equipment, the system either generates a warning signal to the pilot indicating the existence of a potential collision, or generates a warning signal and advises the pilot of the appropriate flight path deviation (conflict resolution advisory) in order to avoid a collision. Systems currently on the market have the capability of recommending vertical escape maneuvers to the pilot. Systems under development will expand this capability by providing both vertical and horizontal resolution advisories to the pilot. These systems are deficient, however, because not all aircraft have ATCRBS transponders and, of those that do, not all have Mode C or Mode S altitude reporting capability.
While the current group of TCAS are installed in large commercial aircraft, high cost has inhibited their widespread adoption in non commercial aircraft. In the United States, there are 184,433 general aviation aircraft and 7,320 air carrier aircraft (1992 data), indicating that the vast majority operate without TCAS.
The current ATCRBS transponder based TCAS also have limitations in areas of high traffic volume when system saturation may occur as the interrogation and transponder signals from different aircraft overlap and intermix. When this occurs, the collision avoidance protection provided by the system is greatly impaired when it is needed most as interrogations are missed or go unanswered.
Recent improvements in the accuracy of radio navigation aids has suggested that they might be used to create an effective anti-collision system. The most precise system currently available to the public is the Global Positioning System (GPS), which consists of a network of 24 satellites orbiting the earth. Each satellite transmits a ranging signal at 1.575 GHz. By monitoring the signal from four or more satellites, a vehicle with a GPS receiver can determine its latitude, longitude, and altitude to an accuracy of 100 meters. (A more accurate signal is available to the military.) The steadily dropping cost of GPS receivers has suggested that it will be used in more applications and greater numbers in the future.
In order for an aircraft collision avoidance system to be most effective, all aircraft should be equipped with a TCAS beacon which transmits their position, altitude, and a unique identifying address automatically, without requiting interrogation. A similar transmitting beacon could also be installed on hazardous obstructions to aircraft such as radio towers, tall buildings and mountain peaks to enable TCAS receiver equipped aircraft to identify and avoid them as well. An additional advantage to this scheme is that position and altitude information from all aircraft would also be available to Air Route Traffic Control Center (ARTCC) and tower controllers as a supplement or alternative to ground based radar. Drug enforcement and stolen aircraft tracking can also be facilitated if an aircraft is observed on radar without the accompanying TCAS beacon transmission containing its unique identifier. It is therefore a desirable goal to produce a simple and inexpensive TCAS beacon transmitter which broadcasts position, altitude, and identification that should be installed on all aircraft as well as an inexpensive TCAS receiver that will be more widely utilized than the present system. Existing ATCRBS transponder based TCAS equipment could be modified to utilize the superior position information of a GPS based system while retaining the current conflict resolution computation and display portions.
The combination of GPS with the development of an anti-collision system for vehicles has been suggested in at least one other reference. U.S. Pat. No. 5,153,836 to Fraughton et al. discloses a collision avoidance system and method that is based on determining the location of aircraft from GPS signals. FIG. 1 shows a general representation of how this system operates.
The Global Positioning System (GPS) satellites 2 emit a 1.575 GHz signal 4 that can be detected with an appropriate receiver. By comparing the signals received from the satellites, a receiver mounted in a vehicle can determine the position and velocity of the vehicle anywhere on the globe. It takes signals from a minimum of four satellites 2 to determine the latitude, longitude, and altitude of an aircraft. Only three satellite signals are required to determine the latitude and longitude of a vehicle.
As shown in FIG. 1, aircraft A and aircraft B each receive precise ranging signals transmitted by the satellites, from which they calculate their actual location (latitude, longitude and altitude). In accordance with the Fraughton et al. patent, aircraft A and B continuously transmit this information, called a navigation solution 6, in an omnidirectional pattern. Each aircraft receives the location information transmitted by the other aircraft and uses its location information plus the received location information to calculate a range vector which indicates the position and distance of the other aircraft relative to their own. For example, if aircraft B were transmitting its location, aircraft A would receive the signal and calculate range vector 10. By monitoring the range vector 10, the receiver of aircraft A can determine when an airspace conflict is about to occur, and notify the pilot of any impending collisions or dangerous situations. Obviously, aircraft B as well as other aircraft (not shown) can do the same.
Although the system proposed in Fraughton et al. is simpler than the FAA system, which relies on transponders and interrogations, it still has inherent drawbacks. Most importantly, if a high number of aircraft exist in the same airspace, the number of transmissions and receptions that must be made can cause the system to saturate. Each aircraft in Fraughton et al. omnidirectionally transmits its navigation solution on a random basis. Before transmitting, each aircraft listens to the transmission channel for a conflicting signal, and then waits a random period of time if the channel is busy. This leads to potentially dangerous situations if the delay in transmission becomes extended due to a busy communications channel.
Fraughton et al. attempts to avoid system saturation by limiting the transmission range of the navigation solution. The strength of the radio frequency transmission is monitored to insure that the transmitted signal is only transmitted with enough strength so that it will be received by aircraft within a predetermined range of interest. Aircraft receiving navigation solutions ignore low strength signals, ensuring that only transmissions from aircraft within a predetermined range of interest--10 miles, for example--are monitored. While this method of avoiding system saturation may work in some environments, in others it is less than effective. For example, in mountainous terrain it is often difficult to judge the distance that a transmitted signal will travel based solely on the strength of the transmission signal. This reduces the reliability of Fraughton et al.-type anti-collision systems as aircraft fail to monitor other aircraft in nearby airspace. As a result, there exists a need for a TCAS system that, while using GPS navigation signals, exhibits an improved method of avoiding system saturation.